Solvents form a class of substances widely used in many economical sectors where they play diverse roles. These are liquids capable of dissolving, diluting or extracting other compounds without generating chemical modifications. However, traditional solvents are general volatile organic compounds, harmful for health and for the environment. Environmental regulations nowadays call for the use of substitution solvents, notably in industrial applications. In this context, novel solvents called biosolvents, stemming from renewable, non-toxic and biodegradable raw materials, have appeared on the market. They have the advantage of providing an alternative to fossil, notably petroleum resources on the one hand and a positive environmental balance on the other hand.
The main biosolvents involved in cleaning formulations, formulations for plant health products, printing inks, paints, varnishes or bituminous binders are esters of fermental organic acids such as ethyl lactate, fatty acid esters, ethanol, terpene derivatives, glycerol or sugars derivatives. Among these biosourced compounds, ethyl lactate and fatty acid esters are known for their solvent properties, used alone or as mixtures. From U.S. Pat. No. 6,284,720, a composition of biosolvents is notably known, comprising from 40 to 70% by weight of a C1-C4 lactic acid ester, preferably ethyl lactate, and from 1 to 30% by weight of a C16-C20 ester of a C1-C4 fatty acid having a melting point below −10° C., preferably ethyl esters (which have the advantage of being 100% biosourced, ethanol being produced by fermentation of sugars while methanol stems from petroleum) or methyl esters of fatty acids from a vegetable oil. The composition may further comprise additives, surfactants . . . . From U.S. Pat. No. 6,191,087, a biosolvent composition is also known comprising from 10 to 60% by weight of a C16-C20 fatty acid ester having a melting point below −10° C., from 20 to 75% by weight of a C1-C4 lactic acid ester, from 0 to 20% by weight of a surfactant, from 0 to 20% by weight of a thickener and from 0 to 50% by weight of an organic solvent. The publication of Datta et al. (Journal of Chemical Technology and Biotechnology, 2006, 81, 1119-1129) describes the different applications of lactic acid and of its derivatives, notably the use of ethyl lactate in compositions of biosolvents.
The compositions of biosolvents are obtained by mixing ethyl lactate with the desired fatty acid esters. Ethyl lactate is produced by esterification of lactic acid with ethanol. However, a major problem of this reaction is that it is balanced. It is therefore necessary to displace the equilibrium for obtaining a suitable yield. This is notably possible by using excess ethanol or by continuously drawing off the water formed during the reaction. The solutions presently set into place for solving this major problem are at the origin of a substantial increase in the cost of ethyl lactate. Another problem is the oligomerization of lactic acid during the reaction.
Many investigations have been conducted for finding a remedy to these problems and to improve the yield in ethyl lactate. From GB 1,282,926, a method for separating acids is notably known, comprising esterification of the acids, separation of the obtained esters and recovery of the corresponding acid. The described method comprises the following steps: an aqueous solution containing lactic acid is mixed with a water-miscible alcohol. The resulting solution is extracted by moderately heating it with an organic solvent non-miscible with water. The lactic acid contained in the aqueous phase is transformed into a lactic acid ester, which ester passes into the organic phase. This organic phase is then distilled in order to retrieve the non-miscible solvent. The residue is then distilled in order to recover the lactic acid ester in a pure form and is then transformed into an acid. With this method it is therefore possible to extract the formed ester from the aqueous phase which displaces the equilibrium towards the formation of the ester. However, the solvents used are non-biological organic solvents and some of them are toxic (benzene, chloroform . . . ).
From U.S. Pat. No. 1,651,666, a method for ester preparation by reaction between a mixture of an alcohol, an acid and a solvent of the corresponding ester is known. However, the solvent of the ester is an oil derived from petroleum with a high boiling point. With this method, it is not possible to obtain a biosolvent composition comprising esters in a single step.
From US 2005/0143599, the use of a solvent derived from petroleum is also known for esterifying and extracting an acid diluted in water. This method is specifically aimed at the separation of acids and does not allow preparation of a biosolvent comprising esters in a single step.
From U.S. Pat. No. 5,723,639, a method for preparing ethyl lactate is known by reacting ammonium lactate with ethanol, applying a pervaporation membrane. This pervaporation membrane gives the possibility of letting through the water and ammonia formed but does not let through the alcohol and ester formed. Thus, one has a continuous extraction of water. The publication of Budd et al. (Ind. Eng. Chem. Res., 2004, 43, 1863-1867) also describes the use of a pervaporation membrane for the esterification reaction between lactic acid and ethanol. However, such membranes are relatively fragile and of a high cost. The method for preparing ethyl lactate by means of this method is therefore not very economical.
From WO 2004/052826, a method for preparing ethyl lactate by esterification between lactic acid and ethanol is known. This method is based on the existence of a water/ethanol azeotrope. During the process, a water-ethanol gas mixture is extracted continuously, this extract is dehydrated and allows recovery of an ethanol flow which may be recycled. Once again, this method requires additional equipment and treatments for displacing the equilibrium, which increases the production costs of ethyl lactate.
Finally, from WO 01/47860, a method for producing ethyl lactate in two steps is known. The first step consists in a transformation, with removal of water, of a composition of lactic acid into an oligomeric composition of lactic acid. This first step allows removal of the water which may be formed. The second step consists in an esterification of all or part of the contained lactic acid, in monomeric, dimeric, oligomeric or polymeric form, by means of a transesterification catalyst. This method has the drawback of being carried out in two steps which increases the reaction times as well as the costs of the method.
Thus, since the filing of patent GB 1,282,926, methods for synthesis of ethyl lactate by esterification have become complicated, leading to two-step methods or to methods using additional equipment or even costly and sensitive equipment (pervaporation membrane).
The compositions of biosolvents comprising ethyl lactate are obtained in 2 steps, a first step for forming ethyl lactate and a second step for mixing this lactate with biosolvents. Such two-step methods substantially increase the cost price of the final composition, further requiring complementary steps, notably for purifying ethyl lactate.